In backlit displays, brightness enhancement films use structures to direct light along the viewing axis, thus enhancing the brightness of the light perceived by the viewer. A representative example of a light directing film is illustrated in FIG. 1. This film 10 may be manufactured from a suitable polymeric material such as acrylic, polycarbonate, UV-cured acrylate, or like material and has a smooth surface 14 and a structured surface 12 opposite the smooth surface. The structured surface 12 includes an array of linear prism elements 16 arranged side by side to form a plurality of peaks 17 and grooves 18 running the length of the film. In use, light incident upon the smooth surface 14 of this film at relatively high incidence angles is refracted at the smooth surface 14 and the structured surface 12 of the film and is redirected toward an axis which is perpendicular to the smooth surface 14. Additionally, light which strikes the structured surface 12 at greater than the critical angle undergoes total internal reflection from both side surfaces, or facets, 20 of a prism element 16 and is directed back, where it may be recycled by reflective elements. By a combination of reflection, refraction and total internal reflection, the film 10 increases the amount of light directed on axis and decreases the amount of light directed at high angles.
A second sheet of light directing film may be placed closely adjacent the first sheet with the prism elements crossed at approximately 90 degrees to further increase the amount of light directed along the viewing axis. FIG. 2 illustrates, in an exploded view, the orientation of crossed sheets of light directing film. In use, the lower, smooth surface 14′ of the upper sheet 10′ can contact, or very nearly contact, the structured surface 12 of the lower sheet 10.
However, if the displays are viewed closely for long periods of time, even very small defects may be detected by the naked eye, and cause distraction for the viewer. For example, “wet-out” occurs when two surfaces optically contact each other, which causes a variation in light intensity across the display surface area. Brighter areas correspond to areas where there is optical coupling and the less bright areas correspond to less optical coupling, and this variation causes a display to have a non-uniform appearance.